The present invention relates to a multilayer polyester sheet having many favorable properties such as heat-sealing property, transparency, heat resistance, hot-water resistance, recyclability, gas barrier properties, impact resistance, scent retainability and ultraviolet screening property. More particularly, the present invention relates to the polyester sheet which is especially suited for manufacturing the containers having an excellent transparency and maintaining a high heat-sealing strength even after retort treatment thereof with hot water.
Polyesters represented by polyethylene terephthalate (PET) are widely used for various types of sheets and containers because of their many advantageous properties such as high mechanical strength, chemical stability, recyclability, transparency, gas barrier properties, moldability, etc. Recently, in view of the solution of the problems of waste disposal and environmental protection, there has been seen a remarkable expansion in use of polyesters for packing pouches, draw-formed containers, etc. for which polyvinyl chloride, polystyrene, polypropylene and the like have been predominantly used in the past. In use of polyesters for packing pouches, generally the produced pouches are cut into a proper size and then, after the stuff to be contained has been packed therein, the pouches are heat sealed. In case of using a polyester for the draw-formed containers, it is first extruded into a sheet by an extruder and then subjected to drawing to form the desired container.
However, the conventional PET sheets or the packing pouches or transparent draw-formed containers using such sheets were not well satisfactory in heat resistance because PET would be used in a non-stretched or lowly stretched state without conducting a treatment for crystallization, and they involved the problem that when the stuff to be contained and/or the ambient atmosphere was high in temperature, there could take place shrinkage or other deformations of the container such as warps or bulges of its flat portion to cause a variation of container capacity.
PET sheets were also unsatisfactory in hot-water resistance and had the problem that when they were applied to uses necessitating long-time contact with hot water such as the retort treatment for sterilization or hot filling or heating of the stuff contained, there would occur whitening or brushing of the sheet due to plasticization mostly resulting from moisture absorption of the sheet.
It has been attempted to subject the PET sheets to a stretching and orienting operation or to a crystallization treatment for improving their heat and hot-water resistance before they were used into packing pouches or transparent draw-formed containers, but in this case there arose the problem that the heat sealing property, transparency and drawing property of the sheets would be deteriorated.
PET copolymerized with cyclohexane dimethanol compound is superior to PET in heat sealing property and impact resistance but is inferior in gas barrier property, thermal stability and solvent resistance. Also, since such a copolymer is substantially equal to PET in heat and hot-water resistance, it is inapplicable, like PET, to uses such a copolymer of the case where retorting or hot filling is performed or the contained stuff is heated.
Polyethylene naphthalate (PEN) is superior to PET in heat resistance, hot-water resistance, gas barrier property and ultraviolet-screening performance but is inferior to PET in impact resistance and also bad in moldability because of high melt viscosity. In PEN, therefore, there has the problems that sheet extrudability is bad and that it is susceptible to heat deterioration and tends to form many by-products such as acetaldehyde in practical use. Also, since the glass transition temperature of PEN is high, heat sealing must be conducted at a high temperature, which may lead to a prolonged cycletime for heat sealing and occurrence of crystallization during heat sealing. PEN is also unsatisfactory in heat-sealing strength. Usually PEN copolymerized with terephthalic acid or the like has been used for improving impact resistance and moldability, but even in this copolymer, the heat-sealing strength is not improved.
As a solution to these problems, it has been proposed to alloy or laminate PET or PEN with other resins or different types of polyester. However, the alloying tends to cause phase separation and consequent whitening of the produced sheet, while the lamination may make it unable to recycle the scraps which is produced in the production line. Thus, it has been hardly possible in either way to obtain a transparent sheet at high productivity.
In view of the above, there has been available no transparent sheet comprising polyester, which is excellent in both moist heat resistance and heat-sealing property, and also recyclable in the production line.
As the result of the present inventors' earnest studies for solving above problems, it has been found that a substantially non-stretched transparent sheet produced by laminating two different types of polyethylene naphthalate resin each having its specific property, are excellent in heat resistance, hot-water resistance, heat-sealing characteristics and transparency, and also recyclable in the production line. The present invention has been attained on the basis of this finding.